Methods for producing porous polymer films by ion beam irradiation and subsequent chemical etching are known (see, for example, Patent Literatures 1 to 3). When a polymer film is irradiated with an ion beam, damage of the polymer chains in the polymer film occurs due to collision with the ions in regions of the polymer film through which the ions have passed. The damaged polymer chains are more susceptible to chemical etching than other regions of the film. Therefore, by chemically etching the ion-beam-irradiated polymer film, pores corresponding to the tracks of the colliding ions are formed in the polymer film, and thus a porous polymer film having the pores is obtained.
Non-Patent Literature 1 discloses uniformization of an ion beam profile by means of a nonlinear focusing method.